1. Field of the Invention
This invention relates to a combustion state-determining system for internal combustion engines, which detects misfires occurring in the engine, to thereby determine a state of combustion of the engine, based on the state of occurrence of misfires.
2. Prior Art
Conventionally, a misfire-determining system for internal combustion engines is known, for example, by Japanese Provisional Patent Publication (Kokai) No. 4-209949, which detects misfires occurring in the engine and determines that the combustion state of the engine is degraded (the engine is in a misfiring state) when misfires have occurred in the engine at a predetermined reference rate or more whenever the engine rotates a predetermined number of times, wherein the predetermined reference rate is set based on the rotational speed and intake pipe absolute pressure of the engine.
In the above conventional system, however, the predetermined reference rate is set on the premise that the engine has been warmed up. As a result, the system has the following inconveniences: That is, when the temperature of the engine is low, a transient misfiring state can sometimes occur due to enrichment of the air-fuel ratio of a mixture supplied to the engine or smoldering of spark plugs thereof. Consequently, the above transient misfiring state can be erroneously determined to be a continuous or steady misfiring state caused by an abnormality of the engine or the misfire-determining system (system abnormality).
In addition, after completion of warming-up of the engine, if the engine is restarted shortly after the engine operation is stopped (high-temperature restarting), fuel present in a fuel supply passage of the engine boils to generate bubbles. The thus generated bubbles cause insufficiency in the amount of fuel supplied to the cylinders, which can result in a transient misfiring state, similarly to the former case. In this case as well, the above conventional system erroneously determines that the engine is in a misfiring state caused by a system abnormality.